1. Field of the Invention
The present invention relates to a circuit board having a resistor, formed thereon with resistor paste, of which the electrical resistance is adjustable, and a method for manufacturing such a circuit board. More specifically, this invention relates to a circuit board having a resistor formed thereon of which the electrical resistance is accurately adjustable without damaging the circuit board.
2. Description of the Related Art
There is a circuit board product, carrying a semiconductor element thereon, in which a resistor is formed in the circuit board by, for example, printing resistor paste such as carbon paste on the circuit board itself. When the resistor is formed by a screen printing method, carbon paste is coated on the circuit board in accordance with a predetermined pattern and heated at a suitable temperature for hardening the carbon paste. At this time, it is difficult to uniformly coat the circuit board with the carbon paste in thickness and width due to variations in the printing process and, also, the resistor may be unevenly heat-treated to result in a variation in electrical resistance value in the resistor.
In such a manner, when the resistor is formed in the circuit board by the printing method, there are many cases in that the designed electrical resistance value of the resistor is not obtainable.
Accordingly, a trimming treatment, using a laser, is widely used so that the electrical resistance value of the resistor is set within a designed tolerance. The laser trimming method for adjusting the resistance value of the resistor is a method for correcting the electrical resistance value of the resistor to a required value by irradiating a laser beam onto the resistor formed on the circuit board to partially narrow the resistor. For example, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2001-085207, a technique for trimming the resistor, with high accuracy and within a short time, has been developed.
However, in a resinous circuit board, there is problem in that the circuit board may be damaged when the resistor is trimmed by irradiation of a laser beam.
Also, there is a problem, in the reliability of the circuit board, in that micro-cracks generated in the resistor during the laser trimming may vary the electrical resistance value of the resistor over time.